1. Field of the Invention
The present invention relates to valves of the type used to communicate the annulus of a well with the interior bore of a well tubing string and, specifically, to a tubing conveyed production valve which is opened to produce well fluids by dropping a weight down the interior of the tubing string from the surface.
2. Description of the Prior Art
As oil and gas well bores are drilled, the integrity of the borehole is preserved, usually by cementing a casing or liner in place in the borehole. The casing or liner is a metal, cylindrical conduit which must be punctured or perforated over the desired interval in order to produce well bore fluids once drilling is complete. A perforating gun which utilizes some form of fired projectile and an explosive charge is used to perforate the casing or liner to begin production from the well. Prior perforating techniques have either utilized tools which were run on a wireline or cable or were tubing conveyed devices which were run on a tubing string to the desired depth in the well bore. The terms "tubing","casing", and "pipe" may be used interchangeably in this discussion to describe metal cylindrical conduits of the type used in the well bore operations which follow.
In tubing conveyed perforating systems, the tubing can be run into position, a packer set to seal off the well bore, and surface wellhead equipment can be installed. The packer setting can be checked by circulating fluid under pressure through the well annulus or through the well tubing string. Once the topside work is completed and tested for safety, the perforating gun can be fired to bring in the well. A tubing valve can also be incorporated in the tubing string which is opened by dropping a weight, such as a cylindrical bar, down the interior of the tubing string. In certain perforating systems, the dropped bar is also utilized to fire a percussion detonator to perforate the casing and bring in the well.
Prior weight actuated production valves of the type described had an outer cylindrical sidewall with an opening therein and an inner mandrel, also containing an opening, which was spaced apart from the outer cylindrical sidewall to provide an annular chamber. A sliding sleeve valve was located within the annular chamber. The annular chamber was at atmospheric pressure and was initially sealed off from the interior of the tubing string by a frangible pin having a hollow interior.
Once the valve was run into position, a bar was dropped from the well surface to sever the frangible pin. It was then necessary to pressure up the interior of the tubing string from the surface. The tubing pressure was communicated through the opening left by the severed pin and acted upon the sliding sleeve in piston-like fashion to move the sliding sleeve upwardly to an open position which communicated the well annulus, annular chamber, and interior of the tubing string.
Certain problems existed with prior art devices of the type described. For instance, it is often desirable to maintain the tubing interior unpressured, or at a lower pressure than the surrounding formation so that the formation would not be over burdened during the perforating operation. Because it was necessary to pressurize the interior of the tubing string, the prior devices could present possible risks to operating personnel. In addition, it was desirable to provide a device which would fail in the open position rather than the closed position if difficulty was encountered during the perforating operation.